1. Field of the Invention
The present invention relates to open pore, microporous ceramic materials and their method of manufacture.
2. Description of Related Art
Porous materials play a particularly important role in a number of chemical processing industries and applications. Separations based on membranes are critical in such fields as chemical recovery, purification and dehumidification. Porous oxides (e.g., clays, silica, alumina and zeolite) are the materials of choice as catalysts or catalyst supports in chemical and petroleum processing reactions such as hydrocracking, hydrodesulfurization, reforming, and polymerization.
With respect to membrane technology, inorganic membranes offer a number of advantages over polymeric membranes which are typically limited to uses at temperatures below about 250xc2x0 C. These include: i) higher operating temperatures, ii) greater structural integrity and hence the ability to withstand higher pressure differentials and backflushing and iii) improved resistance to corrosion. Porous oxide, (e.g., aluminum oxide) and carbon membranes offer some of these characteristics, but advanced materials are still required for improved strength, toughness, structural integrity, temperature stability, water and oxygen resistance, thermal shock resistance, molecular selectivity to small molecules and gases, and high flux.
Similar considerations apply to clay and metal oxide type catalysts or catalyst supports, particularly as relates to stability and thermal shock resistance at temperatures above about 500xc2x0 C.
Ceramic materials of the Sixe2x80x94C, Sixe2x80x94N, Sixe2x80x94Cxe2x80x94N, Sixe2x80x94Bxe2x80x94C, Sixe2x80x94BN, Alxe2x80x94N, Sixe2x80x94Alxe2x80x94N, Bxe2x80x94Alxe2x80x94N and related types appear to offer many of the properties set forth above. However, the solgel synthesis methods typically used to prepare porous oxide membranes or catalyst supports are incompatible with the preparation of ceramics of the type described above because of the need to use water in their preparation. Sintering or reactive sintering of these ceramics likewise produces materials with pore sizes of from about 0.1 to about 1000 microns which are non-uniform and generally too large for effective molecular separation and other uses described above. Chemical vapor deposition can produce microporous ceramic layers, but this tends to be an expensive, high temperature process with limited ability to tailor complex ceramic compositions.
Recently, researchers have discovered improved methods for preparing ceramics using ceramic precursors as starting materials. A ceramic precursor is a material, a chemical compound, oligomer or polymer, which upon pyrolysis in an inert atmosphere and at high temperatures, e.g., above about 700-1000xc2x0 C., preferably above 1000xc2x0 C., will undergo cleavage of chemical bonds liberating such species as hydrogen, organic compounds and the like, depending upon the maximum pyrolysis temperature. The resulting decomposition product is typically an amorphous ceramic containing Sixe2x80x94C bonds (silicon carbide), Sixe2x80x94N bonds (silicon nitride) or other bond structures which will vary as a function of the identity of the ceramic precursor, e.g., Sixe2x80x94Cxe2x80x94N, Sixe2x80x94Nxe2x80x94B, Bxe2x80x94N, Alxe2x80x94N and other bond structures, as well as combinations of these structures. Crystallization of these amorphous ceramic products usually requires even higher temperatures in the range of 1200-1600xc2x0 C.
The pyrolysis of various ceramic precursors, e.g., polycarbosilanes, polysilanes, polycarbosiloxanes, polysilazanes, and like materials at temperatures of 1200xc2x0 C. and higher to produce ceramic products, e.g., silicon carbide and/or silicon nitride, is disclosed, for example, in M. Peuckert et al., xe2x80x9cCeramics from Organometallic Polymersxe2x80x9d, Adv. Mater. 2, 398-404 (1990).
During pyrolysis, preceramic precursors such as described above liberate various gaseous decomposition species such as hydrogen and organic compounds, including methane, higher molecular weight hydrocarbon molecules, and lower molecular weight precursor fragments. These gases tend to coalesce within the preceramic matrix as they form, resulting in a bulking or swelling to form a voluminous mass of low bulk density. These entrained gases can also lead to the formation of smaller gas bubbles within the developing ceramic mass as the preceramic precursor crosslinks and hardens, resulting in a reduced density ceramic having a mesoporous or macroporous closed-cell structure, without development of a significant amount of open celled micropores.
Where dense, non-porous ceramic materials are sought using ceramic precursors yielding high gas volumes, it is often necessary to conduct the pyrolysis over a very long period of time with very gradual incremental temperature increases and/or under vacuum to assist in removal of these gaseous species at temperatures where they are formed.
The present invention provides for amorphous, microporous, ceramic materials having a surface area in excess of 50 m2/gm, preferably in excess of 100 m2/gm, and an open-pore microporous cell structure wherein the micropores have a mean width (diameter) of less than 20 Angstroms and wherein said microporous structure comprises a volume of greater than about 0.015 cm3/gm, preferably greater than 0.05 cm3/gm, of the ceramic. The invention also provides for a preceramic composite intermediate composition comprising a mixture of a ceramic precursor and finely divided particulate material selected from the group consisting of non-silicon containing ceramics, carbon, inorganic compounds having a decomposition temperature greater than 1000xc2x0 C. and mixtures thereof, whose pyrolysis product in inert atmosphere or in an ammonia atmosphere at temperatures of up to less than about 1100xc2x0 C. gives rise to the microporous ceramics of the invention. As used in this application, the term xe2x80x9cnon-silicon containing ceramicsxe2x80x9d is defined to exclude oxide phases. Also provided is a process for the preparation of the microporous ceramics of the invention comprising: a) forming an intimate mixture comprising from greater than 30 up to about 99 parts by weight of a ceramic precursor oligomer or polymer having a number average molecular weight in the range of from about 200 to about 100,000 g/mole and from about 1 to less than 70 parts by weight of particulate material selected from the group consisting of non-silicon containing ceramics, carbon, inorganic compounds having a decomposition temperature greater than 1000xc2x0 C. and mixtures thereof, said particles having a mean particle size of less than about 10 microns, b) gradually heating said mixture in the presence of an inert gas or ammonia gas and in sequential stages with hold times at intermediate temperatures to a maximum temperature in the range of from about 400xc2x0 C. up to less than about 1100xc2x0 C. and over a period of total heating and hold time of from about 5 to about 50 hours to form a microporous ceramic product, and c) cooling said microporous ceramic product.
The microporous ceramics prepared in accordance with this invention generally exhibit a surface area within the range of from about 50 to about 400 m2/gm based on the combined weight of amorphous phase and particles, and amorphous phase micropore volumes of greater than 0.015 up to about 0.17 cm3/g, wherein the volume fraction of micropores in the ceramic product ranges from about 5% to about 32%.
Ceramics produced in accordance with this invention are particularly useful in bulk sorbent applications, as active layers in membrane separation structures and as catalyst supports.
As used herein, the term microporous ceramic refers to a ceramic having a porous structure wherein the pores have a mean width (diameter) of less than 20 Angstroms. This definition and the physical and chemical adsorption behavior of microporous materials are disclosed in S. J. Gregg and K. S. W. Sing, xe2x80x9cAdsorption, Surface Area and Porosityxe2x80x9d, Academic Press, New York, 1982; and S. Lowell and J. F. Shields, xe2x80x9cPowder Surface Area and Porosityxe2x80x9d, Chapman and Hall, New York, 3rd Edition, 1984. This term is to be contrasted with the term xe2x80x9cmesoporousxe2x80x9d which refers to pores having a mean width of greater than 20 Angstroms up to about 500 Angstroms and the term xe2x80x9cmacroporousxe2x80x9d which refers to pores having a mean width greater than about 500 Angstroms. More specifically, the term microporous refers to such structures wherein essentially all of the pores have a width of from about 2 to about 20 Angstroms. The surface area and micropore volume are calculated from the nitrogen adsorption isotherm, which is measured at 77xc2x0 K. using an automated continuous flow apparatus. The total surface area is calculated using the BET method, and the micropore volume and mesopore/macropore surface area are calculated using the T-plot method, as described in the Gregg reference above. Subtraction of the mesopore/macropore surface area from the total surface area gives an estimate of the micropore surface area.
Ceramic precursor materials which are preferred for the purposes of this invention include oligomers and polymers such as polysilazanes, polycarbosilazanes, perhydro polysilazanes, polycarbosilanes, vinylicpolysilanes, amine boranes, polyphenylborazanes, carboranesiloxanes, polysilastyrenes, polytitanocarbosilanes, alumanes, polyalazanes and like materials, as well as mixtures thereof, whose pyrolysis products yield ceramic compositions containing structural units having bond linkages selected from Sixe2x80x94c, Sixe2x80x94N, Sixe2x80x94Cxe2x80x94N, Sixe2x80x94B, Sixe2x80x94Bxe2x80x94N, Sixe2x80x94Bxe2x80x94C, Sixe2x80x94Cxe2x80x94Nxe2x80x94B, Sixe2x80x94Alxe2x80x94Nxe2x80x94C, Sixe2x80x94Alxe2x80x94N, Alxe2x80x94N, Bxe2x80x94N, Alxe2x80x94Nxe2x80x94B and Bxe2x80x94Nxe2x80x94C, as well as oxycarbide and oxynitride bond linkages such as Sixe2x80x94Oxe2x80x94N, Sixe2x80x94Alxe2x80x94Oxe2x80x94N and Tixe2x80x94Oxe2x80x94C. The preferred precursors are those oligomers and polymers having a number average molecular weight in the range of from about 200 to about 100,000 g/mole, more preferably from about 400 to about 20,000 g/mole. The chemistry of these oligomeric and polymeric precursors are further disclosed in the monograph xe2x80x9cInorganic Polymersxe2x80x9d, J. E. Mark, H. R. Allcock, and R. West, Prentice Hall, 1992.
Particularly preferred polysilazanes are those materials disclosed in U.S. Pat. Nos. 4,937,304 and 4,950,381, the complete disclosures of which are incorporated herein by reference. These materials contain, for example, recurring xe2x80x94Si(H)(CH3)xe2x80x94NHxe2x80x94 and xe2x80x94Si(CH3)2xe2x80x94NHxe2x80x94 units and are prepared by reacting one or a mixture of monomers having the formula R1SiHX2 and R2R3SiX2 in anhydrous solvent with ammonia. In the above formulas, R1, R2 and R3 may be the same or different groups selected from hydrocarbyl, alkyl silyl, or alkylamino and X2 is halogen. The preferred polysilazanes are prepared using methyldichlorosilane or a mixture of methyldichorosilane and dimethyl-dichlorosilane as monomer reactants with ammonia. The primary high temperature pyrolysis product ( greater than 1300xc2x0 C.) of this precursor are silicon nitride (Si3N4) and silicon carbide (Sic). These precursors are commercially available from Chisso Corporation, Japan under the trade designations NCP-100 and NCP-200, and have a number average molecular weight of about 6300 and 1300 respectively.
Another class of polysilazane precursors are polyorgano(hydro)silazanes having units of the structure
[(RSiHNH)x(R1SiH)1.5N]1xe2x88x92x
where R1 is the same or different hydrocarbyl, alkylsilyl, alkylamino or alkoxy and 0.4 less than X less than 1. These materials are disclosed in U.S. Pat. No. 4,659,850, the complete disclosure of which is incorporated herein by reference.
Another preferred ceramic precursor is a polysilastyrene having the structure [(phenyl)(methyl)Sixe2x80x94Si(methyl)2]n available under the trade designation xe2x80x9cPolysilastyrene-120xe2x80x9d from Nippon Soda, Japan. This material has a number average molecular weight of about 2000 and the primary pyrolysis products silicon carbide and carbon.
Other preferred ceramic precursors are polycarbosilanes having units of the structure (Si(CH3)2CH2)n and/or (Si(H)(CH3)CH2)n having a number average molecular weight in the range of about 1000 to 7000. Suitable polycarbosilanes are available from Dow Corning under the trade designation PC-X9-6348 (Mn=1420 g/mol) and from Nippon Carbon of Japan under the trade designation PC-X9-6348 (Mn=1420 g/mol). The main pyrolysis product of these materials in an inert atmosphere are silicon carbide and excess carbon.
Vinylic polysilanes useful in this invention are available from Union Carbide Corporation under the trade designation Y-12044. These yield silicon carbide together with excess carbon as the main pyrolysis product.
Suitable polyalazane (alumane) preceramic precursors are those having recurring units of the structure Rxe2x80x94Alxe2x80x94Nxe2x80x94Rxe2x80x2, where R and Rxe2x80x2, are the same or different hydrocarbyl groups (particularly C1-C4 alkyl), and are described in an article xe2x80x9cPolymer Precursors For Aluminum Nitride Ceramicsxe2x80x9d, J. A. Jensen, pp. 845-850, in xe2x80x9cBetter Ceramics Through Chemistryxe2x80x9d, MRS Symposium Proceedings, Vol. 271. The main pyrolysis product of these materials is aluminum nitride.
Other suitable preceramic precursors will be evident to those skilled in the art, particularly those yielding amorphous or crystal-line phases such as SiC, Si3N4, Sixe2x80x94Cxe2x80x94N, Bxe2x80x94N, Sixe2x80x94Bxe2x80x94N, B4Cxe2x80x94BNxe2x80x94C, Sixe2x80x94Bxe2x80x94C, Sixe2x80x94Alxe2x80x94N, Bxe2x80x94Alxe2x80x94N and Alxe2x80x94N pyrolysis products.
The solid particulate material which is mixed with the ceramic precursor material may be in the form a powder having a mean particle size of less than 10 microns or in the form of finely chopped fibers less than 1 mm long and having a mean diameter of less than 10 microns. These particles may comprise non-silicon containing ceramics such as the nitride of aluminum, the nitrides or carbides of boron, molybdenum, manganese, titanium, zirconium, tungsten and other refractory or rare earth metals, as well as ceramics containing a combination of bond linkages such as Bxe2x80x94Alxe2x80x94N, Bxe2x80x94Nxe2x80x94C and Alxe2x80x94Nxe2x80x94B. These particles may have either a crystalline or amorphous atomic structure.
Other types of particles which may be used include carbon in various forms such as carbon black, carbon fibers, natural or synthetic diamond and tubular fullerenes.
Still other types of particles which may be employed include non-ceramic inorganic compounds having a decomposition temperature in excess of 400xc2x0 C., preferably in excess of 500xc2x0 C. to 1100xc2x0 C. These include Periodic Table Group II, III, IV, V, VI, VII and VIII metal and non-metal oxides, hydroxides, sulfides and like compounds such as alumina (aluminum oxide), silica, iron oxides, copper oxides, nickel oxides, titanium dioxide, zinc oxide, magnesium oxide, chromium oxide, calcium oxide and like materials, as well as crystalline silica-aluminates such as clay, silicalite and zeolites such as Zeolite X, Zeolite Y, beta Zeolite, Zeolite L, Zeolites ZSM-5, ZSM-11, ZSM-25 and like materials.
The surface area and the degree of microporosity which can be achieved in the microporous ceramics prepared in accordance with this invention has been found to vary inversely with the mean particle size or mean diameter of the particles which are blended with the ceramic precursor to form the composite intermediate. Where the mean particle size or diameter is large, i.e., 20 microns or greater, the particles tend to settle within the preceramic matrix giving rise to two distinct phases, i.e., a dense phase and a voluminous non-microporous phase containing high and low concentrations of particles respectively. Preferably the particles will have a mean particle size or diameter of less than 10 microns, preferably less than 5 microns and more preferably from about 0.1 to about 2 microns. Where necessary, commercially available materials of larger particle size can be ground by any suitable means, including cryogenic grinding below minus 100xc2x0 C., to achieve non-aggregate, mean particle sizes within these preferred ranges.
Although the factors underlying the development of the microporous, open-celled ceramic structure achieved in accordance with this invention are not completely understood, it is believed that the individual solid particulates dispersed within the molten or glassy preceramic polymer matrix serve to prevent nucleation of large bubbles of decomposition gases which form within the matrix as the temperature increases. The decomposition gases thus more readily escape from the matrix by diffusion, thereby avoiding the development of a voluminous swelling of the ceramic mass. The elimination of molecular species from the ceramic precursor molecules, accompanied by crosslinking, provides a templating effect which thus entrains a significant volume of microporosity and contributes to enhanced surface area of the resulting solidified ceramic mass.
Another factor which has been found to influence both the total surface area and degree of microporosity achieved in the pyrolyzed ceramic of this invention is the amount of ceramic precursor mixed with the additive particles to form the composite intermediate. This level will vary within the range of from greater than 30 parts by weight up to about 99 parts by weight of ceramic precursor and correspondingly from about 1 to less than 70 parts by weight of the particles. Microporous ceramics having a post-pyrolysis surface area in excess of about 150 m2/gm and a micropore volume in excess of 0.03 cm3/gm, preferably in excess of 0.05 cm3/gm, can be achieved when the amount of ceramic precursor mixed with the particles to form the composite intermediate is in excess of 40 parts by weight up to about 80 parts by weight precursor and the balance to 100 parts by weight of particles. The most preferred range is from about 50 to about 70 parts by weight ceramic precursor per corresponding about 30 to about 50 parts by weight of additive particles, since composite intermediates containing this latter ratio of components can yield post-pyrolysis surface areas of greater than 200 m2/gm and micropore volumes of greater than 0.08 cm3/gm.
The microporous ceramic compositions of this invention are prepared by first forming an intimate mixture of the ceramic precursor and the additive particles to provide a composite intermediate, followed by pyrolysis of the composite intermediate under an inert atmosphere or ammonia in sequential stages with hold times at intermediate temperatures to a final temperature in the range of from about 400xc2x0 C. to less than 1100xc2x0 C.
The composite intermediate mixture may be formed by any suitable process which will provide for a uniform dispersion of the particles within the ceramic precursor matrix. Thus, the components may be ground, ball milled or pulverized together in dry form, or the components may be slurry blended by forming a very fine suspension of the additive particles in an organic liquid which is a solvent for the ceramic precursor, dissolving the precursor in the solvent to form a slurry and evaporating the solvent at temperatures of 30 to 80xc2x0 C. at atmospheric pressure or under vacuum to obtain a composite intermediate composed of the preceramic precursor having the particles uniformly dispersed therein. The composite may then be comminuted to provide a particulate molding powder.
Suitable solvents for the solution blending process include aromatic and aliphatic hydrocarbons such as benzene, toluene, and hexane, as well as ethers such as tetrahydrofuran, diethyl ether and dimethyl ether. Where the slurry blending technique is used, the ceramic precursor and particles are preferably added to the solvent at a combined weight ratio within the range of from about 20% to 50% by weight solids. Ultrasonic mixing techniques and/or a suitable disperant can be used to facilitate the formation of a very fine suspension of the particles in the organic solvent.
Prior to pyrolysis, the composite intermediate may be formed into any desired shape such as a pellet, disc, fiber, thin membrane or other three dimensional shape. The dry composite may be shaped using an extruder or a hydraulic press, with or without heat being applied, or by conducting the pyrolysis in a suitable mold cavity containing the composite intermediate. Fibers may be prepared by extruding or spinning a melt or a solvent slurry of the composite intermediate, while thin separation membranes may be formed by applying a melt or solvent slurry of the composite intermediate to the surface of a suitable substrate, such as another ceramic, and subjecting the structure to well known spin or whirl coating techniques to form a uniform, thin coating of the composite intermediate on the surface of the substrate, followed by heating to evaporate off the solvent where solvent is present.
As indicated above, pyrolysis of the composite intermediate is next conducted by heating it under inert flowing gas, e.g., argon, helium or nitrogen, or under flowing ammonia gas, at a controlled rate of temperature, with preferred hold times at intermediate temperatures to maintain uniformity of the ceramic product, and a final hold time at the maximum heating temperature, followed by gradual cooling of the ceramic end product to room temperature. The heating rate may range from about 0.5 to 10xc2x0 C. per minute, more preferably from about 0.5 to 6xc2x0 C. per minute and most preferably from about 0.5 to less than 3xc2x0 C. per minute. Generally speaking, microporous ceramics are formed by gradually heating the composite intermediate to a maximum temperature (Tmax) in the range of from about 400xc2x0 C. to less than about 1100xc2x0 at a heating rate in the range of from about 30xc2x0 C. to 400xc2x0 C. per hour, with various holding times of about 0.5 to about 5 hours at selected temperatures between about 200xc2x0 C. and Tmax. Total combined heating/holding time may range from about 5 to about 50 hours, more preferably from about 8 to about 24 hours. Holding times and temperatures are dictated by ceramic precursor decomposition and reaction kinetics. Hence, they depend on precursor composition and the rate of evolution of specific molecular species at or about the holding temperature, e.g., CH4, H2, higher molecular weight hydrocarbon or H-C-N species or precursor fragments, as reflected by sample weight losses at or about these temperatures. The flow rate of the inert gas or ammonia gas may range from about 100 to about 1000 cc per minute.
In the more preferred embodiment of the invention, pyrolysis is carried out in a heat treating furnace or muffle oven using the following schedule and using flowing inert gas or ammonia throughout:
i) after flushing the furnace with inert gas, e.g., helium, the temperature is first increased to about 200xc2x125xc2x0 C. over a period of 0.5-3 hours, held at that temperature for a period of 0.5-5 hours, preferably 1-2 hours and the temperature then increased;
ii) in the second step, the temperature is increased to about 300xc2x125xc2x0 C. over a time of from about 0.5 to 5 hours, preferably from 1-2 hours and held at that temperature for 0.5-5 hours, preferably 1-2 hours, and the temperature again increased;
iii) in the third step the temperature is increased to Tmax or about 500xc2x125xc2x0 C., whichever is less, over a time period up to about 5 hours, preferably up to 2 hours, and held at that temperature for 0.5-5 hours, preferably 1-2 hours;
iv) in a fourth step where Tmax is above 500xc2x0 C., the temperature is increased to Tmax or about 700xc2x125xc2x0 C., whichever is less, over a time period up to about 5 hours, preferably up to 2 hours, and held at that temperature for 0.5-5 hours, preferably 1-2 hours;
v) in a subsequent step where Tmax ranges between 700xc2x0 C. and 1100xc2x0 C., the temperature is increased to Tmax over a time period of up to 5 hours, preferably 1-3 hours, and held at Tmax for 0.5-5 hours, preferably 1-2 hours.
In the most preferred embodiment of the invention, the composite intermediate is heated as above with a 1-2 hour hold at about 200xc2x0 C., 300xc2x0 C., 500xc2x0 C. and 700xc2x0 C. (and Tmax if Tmax is greater than 700xc2x0 C.), and the pyrolyzed ceramic then allowed to return from Tmax to room temperature while continuing the flow of inert gas or ammonia during the cooling period.
In addition to particle size and quantity of particles present in the composite intermediate, another factor which influences both surface area and the degree of microporosity which can be achieved in the microporous ceramic is the final temperature to which the ceramic is heated. It has been found with respect to most composite intermediates pyrolyzed under inert or ammonia gas that the surface area and degree of microporosity tends to diminish as Tmax approaches 1100xc2x0 C., and tends to be at maximum levels at Tmax of up to about 700xc2x0 C.xc2x1150xc2x0 C. For these reasons, a more preferred heating schedule is such that Tmax ranges from about 500xc2x0 C. to about 850xc2x0 C., more preferably from about 550xc2x0 C. to about 750xc2x0 C.